1. Technical Field
The present invention relates generally to radio frequency (RF) absorbers operable for use in electro-magnetic compatibility (EMC) testing applications; and, more particularly, it relates to a matching network hybrid EMC absorber for use in treating walls and ceiling of a shielded anechoic (non-reflective) EMC test enclosure.
2. Related Art
RF absorbers are commonly used in treating metallic surfaces of a shielded enclosure to produces an ideal test environment for conducting antenna, and radar cross-section measurements in the frequency range of several hundred Mega-Hertz (MHz) up to 100 Giga-Hertz (GHz) in the past four decades. More recently, similar technology has also been employed to conduct RF measurements for EMC compliance of intentional and unintentional radiators (such as mobile communications devices and digital equipment) from as low as 20 MHz up to 40 GHz. A special class of RF absorbers has been developed to accommodate the low frequency test accuracy required by the EMC test specifications developed by the Federal Communications Commission (FCC) and other International EMC Regulatory bodies. This special class of RF absorbers is often referred to generally as EMC absorbers due to its unique design features that differ from the higher frequency absorbers.
Conventional EMC absorbers, installed on the metallic surfaces of EMC test facilities, are typically covered entirely with an absorber material. Common approached dealing with conventional EMC absorber technology includes modification of the shape of the absorber. The first usable type of EMC absorber is of a dielectric material (polyurethane foam impregnated with carbon film through its volumetric entirety) only having a thickness ranging from 2.4 meters to 3.6 meters. Designs and developments have included the varying of the physical size of absorbers as well as varying the physical shape of absorbers in an attempt to accommodate low-frequency absorbing performance required by EMC test applications. In general, the shape of absorbers may be classified as having a substantially pyramid type of shape. Sometimes, the apex of the pyramid type of shape is severed, leaving a flat top that is parallel with the base. This last variation in the shape of an absorber, having a severed apex of the pyramid, has proven to be one of the largest and most divergent changes in absorber design in the past several years.
The predominant approach among designers of absorbers has been focused on the altering of the size and shape of the absorber to accommodate various test needs. In addition, there has been development directed towards using different sub-layers of various materials on which an absorber is laid in an effort to enhance absorber and test system performance. In these conventional approaches, an entirety of an absorber is impregnated, or at least all of at least one side of the absorber. In some applications, these sub-layers constitute dielectric materials, and sometimes the various sub-layers each have different dielectric constants. Due to enormous size of the absorber and dimensions of the shielded enclosure to accommodate the absorbers and required working space, the large dielectric absorbers are often found to be too costly to be used to build an EMC test facility.
In the 1990""s, another type of EMC absorbers using sintered magnetic lossy material in the form of ferrite tile or ferrite grid tiles was introduced to build EMC test facilities. The ferrite and grid tiles are especially effective at lower frequency range. In addition, the physical thickness of the magnetic absorbers can be as thin as several millimeters to a few centimeters. These advantages enable EMC test facilities to be constructed much smaller. However, due to its flat surface, its higher frequency absorbing performance is severely limited to less than 2 GHz. Furthermore, since most ferrite absorbing materials are manufactured using a 10 cmxc3x9710 cm footprint, the presence of air gaps between ferrite tiles on a wall surface substantially degrades its RF absorbing performance at the low frequency end. These two limitations effectively restrict the use of ferrite absorbers to function alone as premium performing RF absorber to construct an EMC test facility in a cost effective manner.
A third type of EMC absorber combining the ferrite magnetic absorber and foam dielectric absorbers was introduced to circumvent the disadvantages of the previous two types of EMC absorbers. This hybrid type of RF absorber is typically designed to have dielectric absorber in front of the ferrite tile absorbers. When designed properly, the dielectric and magnetic absorbers work jointly to help improve the low frequency end. At higher frequency (above 1 GHz), the dielectric absorber, normally shaped like a pyramid, absorbs almost all of the RF energy before it reaches the ferrite material. The hybrid EMC absorbers with premium absorbing performance are typically of a height from approximately 1.0 to 1.8 meters. However, due to the high cost of volumetric contents of its dielectric absorber and ferrite tiles, hybrid absorbers still cost rather high.
Further limitations and disadvantages of conventional and traditional systems will becomes apparent to one of skill in the art through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.
One aspect of the present invention allows the development of a low-cost broadband matching network in front of the ferrite tile absorber so that the ferrite tile absorber with air gaps can be very effective at low frequency range from approximately 20 MHz to 500 MHz. With the help of numerical model, the matching network and tile absorbers still maintain the overall premium performance from 500 MHz to 1 GHz. The lossy contents of the matching network may also be properly designed so that it absorbs a majority of the RF energy before it reaches the flat surface of ferrite tile in the frequency range of approximately 1 to 40 GHz.
Various aspects of the present invention can be found in a matching network hybrid electro-magnetic compatibility absorber to provide improved radio frequency absorbing performance in a frequency range of approximately 20 MHz to approximately 500 MHz. The matching network hybrid electro-magnetic compatibility absorber includes a big element, a small element that is located beneath the big element, the big element comprises a big element surface, the small element comprises a small element surface, a big element coating that covers a predetermined portion of the big element surface, and a small element coating that covers a predetermined portion of the small element surface.
In certain embodiments of the invention, the matching network hybrid electro-magnetic compatibility absorber has a substantially pyramid-like shape, and the predetermined portion of the big element surface includes less than an entirety of the big element surface, and the predetermined portion of the small element surface includes less than an entirety of the small element surface. At least one of the big element coating and the small element coating has a substantially tear drop shape. At least one of the big element coating and the small element coating has a predetermined thickness. The big element and the small element are separated by a predetermined distance. The big element has at least two surfaces, and the at least two surfaces are separated by a distance having a predetermined thickness. The big element coating is made of a first material, and the small element coating is made of a second material. The matching network hybrid electro-magnetic compatibility absorber also includes at least one additional big element coating that covers at least one additional predetermined portion of the big element surface. The at least one additional predetermined portion of the big element surface being less than an entirety of the big element surface.
Various other aspects of the present invention can be found in a matching network hybrid electro-magnetic compatibility absorber to provide improved radio frequency absorbing performance in a frequency range of approximately 20 MHz to approximately 500 MHz. The matching network hybrid electro-magnetic compatibility absorber includes a layer having a surface, and a coating that covers a predetermined portion of the surface.
In certain embodiments of the invention, the coating has a predetermined shape. The layer includes at least one additional surface, and at least one additional coating covers a predetermined portion of the at least one additional surface. The predetermined portion of the at least one additional surface includes less than an entirety of the least one additional surface. The matching network hybrid electro-magnetic compatibility absorber also includes at least one additional layer. The at least one additional layer includes at least one additional surface. The matching network hybrid electro-magnetic compatibility absorber also includes at least one additional coating covers a predetermined portion of the at least one additional surface. The predetermined portion of the at least one additional surface includes less than an entirety of the least one additional surface. The matching network hybrid electro-magnetic compatibility absorber also includes at least two elements, and at least one of the two elements includes the layer. The layer includes at least one additional surface, and a distance between the surface and the at least one additional surface is of a predetermined thickness. The coating has a predetermined thickness, and the predetermined portion of the surface includes less than an entirety of the surface.
Various other aspects of the present invention can be found in a matching network hybrid electro-magnetic compatibility absorber. The matching network hybrid electro-magnetic compatibility absorber includes an absorber having a surface having a coating. The coating includes at least one of a coating type, a coating shape, a coating thickness, and a coating placement. At least one of the coating type, the coating shape, the coating thickness, and the coating placement is varied as a design parameter to permit absorption of radio frequency energy in a frequency range extending from approximately 500 MHz to approximately 40 GHz.
In certain embodiments of the invention, the coating shape has a substantially tear drop shape. The coating covers an entirety of the surface. Alternatively, the coating covers less than an entirety of the surface. The surface includes at least one additional coating that includes at least one of at least one additional coating type, at least one additional coating shape, at least one additional coating thickness, and at least one additional coating placement.
Other aspects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.